1. Field of the Invention
The present invention relates to a sputtering target to be used in the sputtering process for producing electroconductive transparent films which are used as an electroluminescence (EL) displays, liquid crystal displays, plane heaters, touch panels and the like and a process for manufacturing the same.
2. Description of the Background
Recent developments in the technology of making thin films has accelerated the use of electroconductive transparent films in the field of display elements and the like. At present, as materials for making electroconductive transparent films, there are known various oxides such as indium oxide-tin oxide (hereinafter referred to as ITO), tin oxide-antimony oxide, and zinc oxide, metals such as gold, platinum and silver, and non-oxide materials such as chalcogenides, lanthanum boride, and titanium nitride. Among them, however, ITO is the most commonly used because of its stability and high degree of light transmittance. Meanwhile, the electroconductive transparent films from ITO are known to be produced by the vacuum evaporation method, the reactive sputtering process using an indium-tin metal target, and the sputtering process employing a target which contains ITO (hereinafter referred to as ITO target). The sputtering process using the ITO target is the most popular because of the easy control of the formed films.
However, the sputtering is usually conducted on a target with a plasma controlled by a magnet. When a conventional ITO target having a flat plate shape is employed, the target will be eroded according to the shape of said magnet and therefore the utility efficiency of the target is lowered to 20 to 40%, resulting in an inefficient use of the target. This will cause a problem. The utility efficiency may be increased if the used ITO target is recovered, dissolved and purified to obtain a regenerated powder, and a new target is prepared from the regenerated powders. However, the quality of the target prepared with the regenerated powder is inferior to that of the original target.
On the other hand, conventional ITO targets have been prepared by molding powders containing ITO (hereinafter referred to as ITO powder) by a metal press molding process to obtain a molded shape containing ITO (hereinafter referred to as an ITO molding) and then sintering the ITO molding.
However, uniform pressing to obtain an ITO molding is difficult with the metal press molding process. When a larger target, which is required for a sputtering apparatus of a larger scale, is to be prepared, a target thus prepared may warp or crack. Therefore, in order to overcome the problems of warping and cracking, a common practice for preparing large targets is that a plurality of precious metal mold components are prepared for every different shape of an object target to obtain fractions of the object target, and these thus-prepared fractions are joined together into a complete shape. However, the thus-prepared large scale targets still suffer from the problem that extraordinary electric discharges may occur at the joints.
Further, indium oxide and tin oxide are not readily sintered, and therefore, the ITO targets obtained by the above-mentioned method suffer from the disadvantage that the relative density is less than 70%. Targets with a low relative density possess high electric resistance, weak resistance against bending, and low thermal conductivity. When sputtering is conducted with such targets, an extraordinary electric discharge often occurs, which can cause adverse effects on the electroconductive transparent film of the ITO thus prepared.
Thus, there remains a need for sputtering targets which exhibit a high utility efficiency. In addition, there remains a need for preparing sputtering targets of large scale and sputtering targets which possess a high relative density, a low electric resistance, a high resistance to bending, and a high thermal conductivity; and which are not susceptible to warping, cracking, or extraordinary electric discharges.